Apparatus and methods for single photon sources

ABSTRACT

A photon source to deliver single photons includes a storage ring resonator to receive pump photons and generate a signal photon and an idler photon. An idler resonator is coupled to the storage resonator to couple the idler photon out of the storage resonator and into a detector. Detection of the idler photon stops the pump photons from entering the storage resonator. A signal resonator is coupled to the storage resonator to couple out the signal photon remaining in the storage resonator and delivers the signal photon to applications. The photon source can be fabricated into a photonic integrated circuit to achieve high compactness, reliability, and controllability.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 15/172,747, now U.S. Pat. No. 9,798,219, filed Jun. 3, 2016,entitled “Apparatus and Methods for Single Photon Sources,” which claimspriority to U.S. Application No. 62/170,329, filed Jun. 3, 2015,entitled “ON-DEMAND SINGLE PHOTON SOURCE BASED ON DYNAMIC PHOTON STORAGEON A PHOTONIC INTEGRATED CIRCUIT,” which is hereby incorporated hereinby reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with Government support under Grant No.FA9550-14-1-0052 awarded by the Air Force Office of Scientific Research.The Government has certain rights in the invention.

BACKGROUND

Single photon sources, which can deliver one photon at a time upondemand of a user, have a wide range of applications especially inquantum information technologies. For example, in quantum keydistribution, single photons delivered by single photons sources can actas quantum bits (qubits), which store information in their polarizationstate or phase. Suppressing multi-photon states can be important inquantum key distribution, because a multi-photon state is susceptible toa photon number-splitting attack. Single photon sources can also be usedin some implementations of quantum computers, which allow the solutionof problems that cannot be solved efficiently by classical computation.In quantum lithography, the coherence of an n-photon number state, whichcan be produced by combining n single-photon states, can achieve ann-fold increase in the resolution of an interferometric measurement,compared to the Rayleigh resolution limit obtained using a classicalbeam containing n photons. In addition, single photons could also beuseful in performing sensitive absorption measurements (e.g., quantumradiometry).

Existing single photon sources attempt to deliver single photons viavarious mechanisms, but each has its own drawbacks. For example,attenuated coherent light (e.g., a laser), which obeys Poissonstatistics, may emit single photons by tuning the mean photon number tobe one from a statistical point view. However, the fluctuations aboutthat mean photon number can impair the repeatability of the source. Inanother example, single quantum dots in III-V and II-VI semiconductorheterostructures, or single trapped atoms and ions, can also be used assingle-photon emitters, but most of these sources operate at cryogenictemperatures. Color centers combined with vacancy centers in diamond,such as N-vacancy centers, Ni—N complexes, Si-vacancy centers, andXe-vacancy centers may also be employed to emit single photons, but thecollection efficiency of these diamond-based sources is generally verylow.

SUMMARY

Embodiments of the present invention include apparatus, systems, andmethods of producing signal photons. In one example, a single-photonsource includes a storage resonator to receive pump photons from aphoton source at a pump frequency ω_(P) and to generate a signal photonat a signal frequency ω_(s) and an idler photon at an idler frequencyω_(i) from the pump photons. The signal frequency ω_(s) is differentthan the idler frequency ω_(i). The single-photon source also includes adetector, operably coupled to the storage resonator, to detect the idlerphoton and to generate a control signal in response to detection of theidler photon. A switch is operably coupled to the detector and opticallycoupled to the storage resonator to prevent transmission, in response tothe control signal, and to allow transmission, in response to a clocksignal, of subsequent pump photons from the photon source to the storageresonator. A signal resonator is optically coupled to the storageresonator to receive the signal photon out of the storage resonator and,in response to the clock signal, to couple the signal photon into anoutput coupler.

In another example, a method of delivering single photons includescoupling at least two pump photons at a pump frequency from a photonsource to a storage resonator. A signal photon at a signal frequency andan idler photon at an idler frequency different than the signalfrequency are the generated in the storage resonator from the at leasttwo pump photons. The method also includes detecting the idler photonwith a detector and preventing transmission of subsequent pump photonsfrom the photon source to the storage resonator in response to detectionof the idler photon in. The method further includes coupling the signalphoton out of the storage resonator in response to a clock signal.

In yet another example, an apparatus to deliver single photons includesa pump ring resonator to receive pump photons at a pump frequency ω_(P)from a photon source. The pump resonator is configured to resonate atthe pump frequency ω_(P). A storage ring resonator is optically coupledto the pump ring resonator, to receive the pump photons from the pumpring resonator and to generate a signal photon at a signal frequencyω_(s) and an idler photon at an idler frequency ω_(i) different than thesignal frequency ω_(s). The storage ring resonator is configured toresonate at the signal frequency ω_(s). An idler ring resonator isoptically coupled to the storage ring resonator to receive the idlerphoton generated in the storage ring resonator, wherein the idler ringresonator is configured to resonate at the idler frequency ω_(i). Theapparatus also includes a photon detector, operably coupled to the idlerring resonator, to detect the idler photon from the idler ring resonatorand to generate a control signal in response to detection of the idlerphoton. A switch is operably coupled to the photon detector andoptically coupled to the pump ring resonator, to prevent transmission,in response to the control signal, and to allow transmission, inresponse to a clock signal, of subsequent pump photons from the photonsource to the pump ring resonator. The apparatus further includes asignal ring resonator, optically coupled to the storage ring resonator,to resonate at the signal frequency ω_(s) in response to the clocksignal and to couple the signal photon out of the storage ringresonator.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A shows a schematic of an on-demand single photon source usingring resonators.

FIG. 1B shows mode spectra of different ring resonators in thesingle-photon source shown in FIG. 1A.

FIG. 2A shows a schematic of a coupler that can be used in thesingle-photon source shown in FIG. 1A.

FIG. 2B shows transmission spectra of the coupler shown in FIG. 2A atdifferent phase delays.

FIG. 3A shows a schematic of a single-photon source using frequencyselective tunable gates.

FIG. 3B shows an example protocol to release a single photon in thesingle-photon source shown in FIG. 3A.

FIG. 4A shows fidelity of the single-photon source shown in FIG. 3A as afunction of the detection time.

FIG. 4B shows maximum success probability of the single-photon sourceshown in FIG. 3A as a function of release time.

FIG. 5 illustrates a method of producing single photons.

DETAILED DESCRIPTION

Single Photon Sources Using Ring Resonators

In applications of single photon sources (SPSs), it is usually desirablefor the photon sources to have the following properties. First, thesingle photon is produced at a specific time, or within a definite timebin (i.e., an “on-demand” SPS). On-demand SPSs usually use a pulsedexcitation source and can be particularly useful in quantum keydistribution and quantum computation, where qubits are typicallyreceived at definite time intervals. Second, it can be desirable for theSPS to be efficient. The efficiency of the photon source producing asingle photon, given one optical or electrical excitation, is desired tobe as close to unity as possible. Third, for quantum informationapplications, it is helpful for the SPS to produce photons with adefinite polarization, since introducing a polarizer after theunpolarized source may waste, on average, half the photons. In addition,it can also be desirable for single photon sources to have a high bitrate, operate at room temperature, and be robust and easy to implement.

To achieve at least some of the above desirable properties in singlephoton sources, apparatus and methods described herein employ a photonicintegrated circuit (PIC) approach that integrates non-classical lightsources with optical gates and detectors to produce single photons. Acombination of heralding and dynamic photon storage is employed toachieve on-demand single photon emission on CMOS-compatible PICs. Theoptical gates and dynamic photon storage are realized using ringresonators, which can be manufactured using semiconductor fabricationtechniques and integrated into a compact circuit. Compared toconventional photon sources using bulk optics, the PIC single photonssources, can have a much smaller form factor, improved mechanicalstability (e.g., more robust against misalignment), and bettercontrollability (e.g., using modulation of the ring resonator to controlthe timing).

FIG. 1A shows a schematic of a photon source 100 using ring resonatorsfor optical gating and dynamic photon storage and delivery. The photonsource 100 includes a light source 105 to provide pump light atfrequency ω_(p). The pump light can be either continuous wave (CW) orpulsed. The pump light propagates into a switch 130, which can transmitor block the pump light, depending on the control signals received bythe switch 130. The switch signals come from two sources: one is aclock-signal from a clock 102 and another is from a photon detector 120(to be described below). A pump resonator 150 is coupled to the switch130 (e.g., via evanescent coupling to a waveguide connected to theswitch 130) to receive the pump light and further transmit the lightbeams to a storage resonator 110. The pump resonator 150 can also couplepump light out of the storage resonator 110 when, for example, theswitch 130 is off (blocking pump light from entering the pump resonator150). In this case, the pump resonator 150 functions as a photon sink tocouple excess pump photons out of the storage resonator 110.

The storage resonator 110 can have a resonant frequency substantiallyequal to the pump frequency ω_(p) to trap the pump light for generatingsignal photons and idlers photons. In one example, the storage resonator110 can trap two pump photons at pump frequency ω_(p) to generate onesignal photon 112 at signal frequency ω_(s) and one idler photon 114 atidler frequency ω_(i) via four wave mixing (FWM) based on Kerrnonlinearity. The three frequencies can satisfy the preservation ofenergy: 2ω_(p)=ω_(s)+ω_(i). Using resonators to generate signal-idlerphoton pairs can have high nonlinear efficiency by effectively recyclingthe optical pump power within the cavity. The nonlinear efficiency candepend on, for example, the quality factor of the storage resonator 110.In on example, the quality factor of the storage resonator 110 can begreater than 5,000. In another example, the quality factor of thestorage resonator 110 can be greater than 10,000. In yet anotherexample, the quality factor of the storage resonator 110 can be greaterthan 20,000.

The storage resonator 110 is further coupled with two more resonators.An idler resonator 160, resonating at the idler frequency ω_(i), isevanescently coupled to the storage resonator 110 to couple out theidler photon 114. The idler resonator 160 further transmits the idlerphoton 114 to the photon detector 120. Upon detection of the idlerphoton 114, the detector 120 generates a control signal to close theswitch 130 to block further transmission of pump light into the storagering resonator 110. The processes of coupling out the idler photon 114,detecting the idler photon 114, generating the control signal, andclosing the switch 130 can be fast enough such that the switch 130 isclosed before the generation of second pair of signal and idler photons.In this case, after the coupling out of the idler photon 114, thestorage resonator 110 may have a single signal photon 112 circulatingwithin the resonator. If there are excess pump photons (e.g., pumpphotons that are not converted into signal and idler photon pairs) inthe storage resonator 110 after the closing of the switch 130, the pumpresonator 150 can couple them out to clean the storage resonator 110.

A signal resonator 140 is also evanescently coupled to the storageresonator 110. The signal resonator 140 includes a modulator 145 thatcan modulate the resonant frequency of the signal resonator 140. Forexample, without modulation, the signal resonator 140 can have aresonant frequency ω_(s0). Upon receiving a clock signal (e.g., a manualcontrol signal or an automatic one), the modulator 145 can change theresonant frequency to ω_(s) of the signal resonator 140 so as to coupleout the signal photon 112 circulating in the storage resonator 110 andtransmit the signal photon 112 to an output waveguide 145 forapplications. After the delivery of the signal photon 112, a new processof single photon generation can start from opening the switch 130 andtransmitting pump photons into the storage resonator 110 forsignal-idler photon generation.

FIG. 1B shows mode spectra of the different ring resonators in thephoton source 100 shown in FIG. 1A. Using the photon source 100 withreference to the mode spectra, one example photon emission cycle caninclude the following steps. First, at least two pump photons are sentinto the storage ring 110, where a pair of a signal photon 112 and anidler photon 114 is generated. Then, the idler photon 114 of thesignal-idler pair is coupled to the detector 120 via the idler resonator160, which can be resonant with ω_(i). In response to sensing the idlerphoton, the detector 120 triggers the switch 130 to stop the pump laserfrom entering the pump resonator 150. Any pump photons remaining in thestorage resonator 110 can be coupled out via the pump resonator 150 toreduce the probability of generating additional photon pairs. The signalphoton 112 is kept in the storage resonator 110 until a clock-signalarrives at the signal resonator 140, causing the signal resonator 140 totemporarily tune into resonance with ω_(s) to release the signal photon112, before returning to its uncoupled state. The clock-signal alsoarrives at the switch 130, which responds by returning to its openstate, completing the cycle.

The degree of determinism in the photon emission of the photon source100 can depend on the intrinsic lifetime of the storage resonator 110.To increase the determinism, the quality factor of the storage resonator110 can be substantially greater than the detector jitter, the lifetimesof the three modes (ω_(p), ω_(i), and ω_(s)) when resonant with theircorresponding access resonators (pump resonator 150, idler resonator160, and signal resonator 140, respectively), as well as the operationtime of the control elements. In one example, lifetimes of about 10 nscan be achieved in silicon resonators, thereby making state of the artelectrical control components with operating speeds of about 10 ps toabout 100 ps sufficient. This can also allow for integration of on-chipelectrical control in the CMOS fabrication of large-scale PICs.Furthermore, the controllable release of photons can enable pulseshaping of the wave packets, which can in turn influence the performanceof quantum information processing systems.

The entire photon source 100 can be fabricated in a single substrate 101(also referred as a single chip) to decrease the size, thereby improvingthe miniaturization. Various thin-film based platforms can be employedto fabricate the apparatus 100. In one example, the apparatus 100 can befabricated on a silicon-on-insulator (SOI) platform. In another example,the apparatus 100 can be fabricated on a lithium niobate platform (alsoreferred to as lithium niobate-on-insulator platform). In yet anotherexample, the apparatus 100 can be fabricated on an aluminum nitride(AlN) platform. In yet another example, the apparatus 100 can befabricated on a silicon nitride platform.

The resonators, including the storage resonator 110, the signalresonator 140, the pump resonator 150, and the idler resonator 160, canbe ring resonators that can be fabricated using existing semiconductorfabricated techniques. Various materials may be used to form the ringresonators (and the output waveguide 140), such as silicon, germanium,silicon oxide, silicon nitride, and chalcogenide glass.

The diameters of the ring resonators may depend on, for example, thedesired resonance wavelength (Δ=ω/c, where ω is the resonant frequencysuch as ω_(p), ω_(i), and ω_(s)) and/or the desired number of supportedlongitudinal modes. As understood in the art, the resonance wavelength λof the mth mode in a ring resonator is λ=Dπn_(eff)/m, where D is theresonator diameter, n_(eff) is the effective refractive index (RI) ofthe mth mode of the ring resonator, and m is an integer. In one example,the ring resonators support only one longitudinal mode (m<1 for a givenD and λ). In another example, the ring resonators support multiplelongitudinal modes. The resonance wavelength of the ring resonators canbe about 1 μm to about 40 μm (e.g., 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25μm, 30 μm, 35 μm or 40 μm). In another example, the resonant wavelengthof the ring resonators can be similar to those used in opticalcommunications (including the internet), at wavelengths between about1.4 μm and about 1.7 μm (e.g., 1.4 μm, 1.5 μm, 1.55 μm, 1.6 μm, 1.65 μm,and 1.7 μm). In practice, the diameter D of the ring resonators can beabout 5 μm to about 150 μm (e.g., 5 μm, 7.5 μm, 10 μm, 15 μm, 20 μm, 25μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120μm, 130 μm, 140 μm, or 150 μm). The cross section of the ring resonatorscan have various shapes, such as round, oval, rectangular, square, orany other shape known in the art. In one example, the cross section ofthe ring resonators can have a size that is about 220 nm by 500 nm.

The detector 120 coupled to the idler resonator 160 is employed todetect the idler photon 114 and therefore monitor the photon pairgeneration within the storage resonator 110. In practice, it can behelpful for the detector 120 to have the following properties: 1) highdetection efficiency, i.e., high probability that a photon issuccessfully detected every time it hits the detector; 2) low darkcurrent, i.e., low probability that the detector registers a photon whennone is there; 3) low reset or “dead time”, i.e., a short interval aftera detection during which the device cannot detect a new photon; 4) lowcross-talk, i.e., low probability that neighboring pixels detect photonsarising from the detection process in a given pixel; and 5) low “timingjitter”, i.e., low uncertainty in specifying when a photon arrives.

In one example, the detector 120 can include a single avalanchephotodiode (APD). In another example, the detector 120 includes an arrayof APDs, which are reverse-biased variants of p-n junction photodiodes.Typically, one pixel includes one APD, one biasing circuit, one timingcircuit, and an interface to the readout circuitry (e.g., shiftregisters) for the array. Without being bound any particular theory ormode of operation, reversely biasing a p-n junction photodiode cangenerate an electric field in the vicinity of the junction. The electricfield tends to keep electrons confined to the n side and holes confinedto the p side of the junction. Absorption of a photon having sufficientenergy (e.g., >1.1 eV for silicon) can produce an electron-hole pair.The electron in the electron-hole pair drifts to the n side and the holedrifts to the p side, resulting in a photocurrent flow in an externalcircuit.

The same principle also allows an APD to detect light. However, an APDis typically designed to support high electric fields to facilitateimpact ionization. More specifically, the electron and/or the hole in anelectron-hole pair generated by photon absorption can be accelerated bythe high electric field, thereby acquiring sufficient energy to generatea second electron-hole pair by colliding with the crystal lattice of thedetector material. This impact ionization can multiply itself many timesand create an “avalanche” of electron-hole pairs. A competition candevelop between the rate at which electron-hole pairs are beinggenerated by impact ionization and the rate at which they exit thehigh-field region and are collected. The net result can be dependent onthe magnitude of the reverse-bias voltage: if the magnitude is below aparticular value (commonly known as the breakdown voltage), collectionnormally outruns generation, causing the population of electrons andholes to decline. An APD operating in this condition is normallyreferred to as a linear mode APD. Each absorbed photon normally createson average a finite number M (also referred to as the internal gain) ofelectron-hole pairs. The internal gain M is typically tens or hundreds.

While M might be the average number of electron-hole pairs generated byone absorbed photon, the actual number may vary, inducing gainfluctuations. This gain fluctuation can produce excess noise, ormultiplication noise, which typically gets progressively worse withhigher M. Therefore, once the point is reached where the multiplicationnoise dominates over the noise introduced by downstream circuitry,further increases in gain may reduce the system's signal-to-noise ratio(SNR). The multiplication noise can also depend on material propertiesbecause, in general, electrons and holes have different likelihoods ofinitiating impact ionizations. For example, in Si, electrons tend to bemuch more likely to impact ionize compared to holes. Therefore, it canbe helpful for electrons to initiate impact ionization in silicon-basedAPDs.

In another example, the detector 120 can include an APD operating inGeiger mode (also referred to as a Geiger-mode APD or GmAPD). A GmAPDoperates when the reverse biased voltage is above the breakdown voltage.In this case, electron-hole pair generation normally outruns collection,causing the population of electrons and holes in the high-field regionand the associated photocurrent to grow exponentially in time. Thegrowth of photocurrent can continue for as long as the bias voltage isabove the breakdown voltage.

A series resistance in the diode, however, can limit the current growthby increasing the voltage drop across the series resistance (therebyreducing the voltage across the high-field region) as the current grows.This effect can therefore slow the rate of growth of the avalanche.Ultimately, a steady-state condition can be reached in which the voltageacross the high-field region is reduced to the breakdown voltage, wherethe generation and extraction rates balance. Stated differently, theseries resistance can provide negative feedback that tends to stabilizethe current level against fluctuations. A downward fluctuation incurrent can cause a decrease in the voltage drop across the seriesresistance and an equal increase in the drop across the APD high-fieldregion, which in turn increases the impact-ionization rates and causesthe current to go back up.

The quenching circuit of the APD employed for the photon source 100 canbe either passive or active. In a passive-quenching circuit, the APD ischarged up to some bias above breakdown and then left open circuited.The APD then discharges its own capacitance until it is no longer abovethe breakdown voltage, at which point the avalanche diminishes. Anactive-quenching circuit actively detects when the APD starts toself-discharge, and then quickly discharges it to below breakdown with ashunting switch. After sufficient time to quench the avalanche, theactive-quenching circuit then recharges the APD quickly using a switch.

In yet another example, the detector 120 can include an array ofsuperconducting nanowire single-photon detectors (SNSPDs), each of whichtypically includes a superconducting nanowire with a rectangular crosssection (e.g., about 5 nm by about 100 nm). The length is typicallyhundreds of micrometers, and the nanowire can be patterned in compactmeander geometry so as to create a square or circular pixel with highdetection efficiency. The nanowire can be made of, for example, niobiumnitride (NbN), tungsten silicide (WSi), YBa₂Cu₃O_(7-δ), or any othersubstrate material known in the art.

In operation, the nanowire can be maintained below its superconductingcritical temperature Tc and direct current biased just below itscritical current. Without being bound by any particular theory of modeof operation, incident photons having sufficient energy to disrupthundreds of Cooper pairs in a superconductor can form a hotspot in thenanowire. The hotspot itself typically is not large enough to span theentire width of the nanowire. Therefore, the hotspot region can forcethe super-current to flow around the resistive region. The local currentdensity in the sidewalks can increase beyond the critical currentdensity and form a resistive barrier across the width of the nanowire.The sudden increase in resistance from zero to a finite value generatesa measurable output voltage pulse across the nanowire.

Various schemes can be employed in SNSPD to improve the detectionperformance. In one example, the SNSPD can employ a large area meanderstrategy, in which a nanowire meander is written typically across a 10μm×10 μm or 20 μm×20 μm area to increase the active area and improve thecoupling efficiency between the incident photons and the SNSPD. Inanother example, the SNSPD can include a cavity and waveguide integrateddesign, in which a nanowire meander can be embedded in an optical cavityso as to further increase the absorption efficiency. Similarly, ananowire can be embedded in a waveguide so as to provide a longinteraction length for incident photons and increase absorptionefficiency. In yet another example, ultra-narrow nanowires (e.g., 20 nmor 30 nm) can be employed to construct the nanowire meander so as toincrease the sensitivity to low-energy photons.

In yet another example, the detector 120 can include a transition edgesensor (TES), which is a type of cryogenic particle detector thatexploits the strongly temperature-dependent resistance of thesuperconducting phase transition.

The modulator 145 in the signal resonator 140 can be used to control therelease of the signal photon 112 by tuning the resonant wavelength ofthe signal resonator 140 (e.g, red-shifting the cavity resonance). Themodulator 145 used for the signal resonators 140 can be based on variousmechanisms, depending on, for example, the desired form factor, dynamicrange of modulation, power consumption, etc. In one example, themodulator 145 can include a piezo-electric element or other suitableelement configured to apply a mechanical force to the signal resonator140 so as to modulate the refractive index of the signal resonator 120.The mechanical force can be applied via compression, bending,stretching, shearing, or any other means known in the art.

In another example, the modulator 145 can be configured to apply anelectric field to the signal resonator 140 so as to modulate therefractive index of the signal resonator 140. For example, the modulator145 may apply the electric field via two electrodes, with one electrodeattached to the top of the signal resonator 140 and the other electrodeattached to the bottom of any substrate supporting the signal resonator140. Alternatively or additionally, the electrodes can be attached to aperimeter of the storage resonator 120.

In yet another example, the modulator 145 can be configured to vary atemperature of the signal resonator 140. For example, the modulator 145can include a semiconductor heater fabricated in thermal communicationwith (e.g., beside) the signal resonator 140. In another example, themodulator 145 can include a semiconductor heater fabricated beneath thesignal resonator 140 or within the signal resonator 140, heating thesignal resonator 140 to change the resonant wavelength of the signalresonator 140.

In yet another example, the modulator 145 is configured to apply anacoustic field to the signal resonator 140 so as to modulate therefractive index of the signal resonator 140. In other examples, themodulator 145 can be configured to apply a magnetic field to the signalresonator 140 so as to modulate the refractive index of the signalresonators 140.

If the signal resonator 140 comprises chalcogenide glass, the modulator145 can apply an optical field on the signal resonator 140 so as tomodulate the refractive index of the chalcogenide glass. As understoodin the art, chalcogenide glasses can exhibit several photo-inducedeffects, including photo-crystallization, photo-polymerization,photo-decomposition, photo-contraction, photo-vaporization,photo-dissolution of metals, and light-induced changes in local atomicconfiguration. These changes are generally accompanied by changes in theoptical band gap and therefore optical constants. In addition,chalcogenide glasses also have strong third-order nonlinear effects.Therefore, a modulator comprising chalcogenide glass can adjust theoptical properties of the signal resonator 140 by applying a modulatingoptical field (separate from the light circulating in the signalresonator 140) to the signal resonator 140.

Ring-Ring Coupling Gates

In the photon source shown in FIG. 1A, the storage resonator 110 isevanescently coupled to other resonators, such as the signal resonator140, the pump resonator 150, and the idler resonator 160. Alternativelyor additionally, the coupling between two ring resonators can beachieved by a coupling structure including a nested-ring Mach-Zehnderinterferometer.

FIG. 2A shows a schematic of a coupling structure 200 that can be usedin ring-based photon sources. The structure 200 includes a storage ring210, which can be substantially similar to the storage resonator 110shown in FIG. 1A and described above. A second ring 220 (e.g., thesignal resonator 140, the pump resonator 150, or the idler resonator160) is evanescently coupled to the storage ring 210. A waveguide 230,including an input portion 232 and an output portion 234 is evanescentlycoupled to the second ring 220. A phase shifter 240 is disposed in thewaveguide 230.

FIG. 2B shows transmission spectra of the structure when the phaseshifter 230 applies different amounts of phase shift. The input lighthas central wavelength at about 1550 nm. As seen from FIG. 2B, when a 7phase shift is applied by the phase shifter 240, the structure 200 is ina closed state and the transmission spectrum has absorption peak at thecentral wavelength 1550 nm. When the phase shifter 240 applies any otheramount of phase shift, the structure 200 is in an open state andtransmits light at 1550 nm. More information on ring-ring coupling canbe found in S. Darmawan, et al., Nested-Ring Mach-Zehnder Interferometerin Silicon-on-Insulator, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 20, No.1, 9 (2008), which is hereby incorporated herein by reference in itsentirety.

Single Photon Sources Using Frequency Selective Tunable Gates

The photon source 100 shown in FIG. 1A uses ring resonators to couplelight (including pump light, idler photons, and signal photons) into orout of the storage resonator. In practice, this coupling can be achievedby general frequency selective tunable gates.

FIG. 3A shows a schematic of a photon source 300 using frequencyselective tunable gates. The photon source 300 includes a storage ring310, which can be an ultrahigh Q micro-cavity with a nonlinearity χ⁽²⁾or χ⁽³⁾ that allows signal and idler photons to be generated in pairs. Apump gate 320 couples the storage ring 310 to a driving laser 305. Thepump gate 320 can control the addition of photons into the storage ring310. A detector gate 330 couples the storage ring 310 to a detector 340,which can generate a control signal to control the pump gate 320 as wellas the detector gate 330 via a feedback link 335. The detector gate 330can control the subtraction of photons in the storage ring 310. A signalgate 350 couples the storage ring 310 to an output waveguide 355, whichdelivers the single photos to applications. In addition, solid lines inFIG. 3A represent optical waveguides, while dashed lines representelectrical control signals.

The operation of the photon source 300 can be divided into generationpart and release. During generation, the detector gate 330 is open onlyto idler photons (κ_(iD)≠0 and κ_(sD)=0, where κ_(iD) and κ_(sD) are thetransmission ratio of the idler photon and signal photon, respectively).Generation ends by turning off the pump (i.e., κ_(pP)→0). Release beginsby opening the detector gate 330 to signal photons (i.e., κ_(sD)≠0) whenthere are no more idler or pump photons in the storage ring 310.

FIG. 3B shows an example protocol for release, where the detector gate330 coupling is turned off upon detection of the (N−1)th signal photonwith N being the number of photons in the cavity after generation. Forany protocol, the detector gate 330 coupling can be turned off beforethe release time, T_(R), to allow the final state to exit through thesignal gate. The shape of κ_(sD)(t) can be optimized to increase theprobability that one photon remains in the cavity at T_(R) under theconstraint that the fidelity, defined as the conditional probability P(1 photon|N−1 detected), exceeds a given threshold T_(p). The drivingprotocol in FIG. 3B also describes the time dependence of the pump gatecoupling, κ_(pP)(t), and the detector gate coupling, κ_(sD)(t), inresponse to detection events. Information from the detector 340 can becontinuously fed back to the detector gate 330 to update the protocol inorder to optimize the probability of producing the desired quantumstate. This usage of “real-time” measurement information can be similarto adaptive quantum tomography, where each experiment in a series ofmeasurements can be designed using the knowledge obtained from previousexperiments.

FIG. 4A shows fidelity as a function of the detection time for athreshold value of 90% in the photon source shown in FIG. 3A. A flatfidelity curve indicates that the detector gate 330 coupling is beingdriven optimally. FIG. 4B shows the maximum success probability as afunction of release time for different N, which is the number of photonsin the cavity after generation.

Methods of Delivering Single Photons

FIG. 5 illustrates a method 500 of producing single photons usingresonators for dynamic storage and delivery. The method 500 includesstep 510, at which pump photons (e.g., at least two pump photons) arecoupled into a storage resonator. The storage resonator can includenonlinear materials to allow the generation of a signal photon and anidler photon from two pump photons via four wave mixing at step 520. Thegenerated signal photon and idler photon can circulate in the storageresonator until the idler photon is coupled out (e.g., using a frequencyselective tunable gate) and detected at step 530. Upon detection of theidler photon (i.e., indication that a signal-idler pair has beengenerated in the storage ring), the method 500 proceeds to step 540,where pump photons are blocked from entering the storage resonator so asto reduce the chance of generating more than one pair of signal-idlerphotons. At step 550, the signal photon circulating in the storageresonator is coupled out and delivered to its destinations. At step 560,the transmission of pump photons is resumed and new pump photons arecoupled to the storage resonator for generating signal-idler pairs. Themethod 500 therefore goes back to step 510 and starts another cycle ofsingle photon production.

CONCLUSION

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of designing and making the technologydisclosed herein may be implemented using hardware, software or acombination thereof. When implemented in software, the software code canbe executed on any suitable processor or collection of processors,whether provided in a single computer or distributed among multiplecomputers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes (outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e., “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The invention claimed is:
 1. A single-photon source comprising: a firstresonator to receive pump photons from a photon source and to generate asignal photon from the pump photons; a switch, optically coupled to thefirst resonator, to prevent transmission, in response to a first signal,and to allow transmission, in response to a second signal, of subsequentpump photons from the photon source to the first resonator; and a secondresonator, optically coupled to the first resonator, to receive thesignal photon out of the first resonator and, in response to the secondsignal, to couple the signal photon out of the first resonator.
 2. Thesingle-photon source of claim 1, wherein the first resonator comprises aring resonator.
 3. The single-photon source of claim 1, wherein thefirst resonator is configured to resonate at a first frequency and thesecond resonator is configured to resonate a second frequency differentthan the first frequency.
 4. The single-photon source of claim 1,wherein the photon source comprises a laser source, in opticalcommunication with the switch, to provide the pump photons.
 5. Thesingle-photon source of claim 1, wherein the second resonator isconfigured to resonate at a frequency of the signal photon in responseto the second signal so as to couple the signal photon out of the secondresonator.
 6. The single-photon source of claim 1, wherein the secondresonator is fabricated in a semiconductor substrate.
 7. Thesingle-photon source of claim 1, further comprising: a third resonator,optically coupled between the switch and the second resonator, to couplethe pump photons from the photon source into the first resonator.
 8. Thesingle-photon source of claim 1, further comprising: a detector,operably coupled to the first resonator, to detect an idler photongenerated from the pump photons and to generate the control signal inresponse to detecting the idler photon.
 9. The single-photon source ofclaim 1, further comprising: a clock signal generator, operably coupledto the second resonator, to generate the second signal.
 10. A method ofdelivering single photons, the method comprising: coupling pump photonsat a pump frequency from a photon source to a resonator; generating asignal photon at a signal frequency different than the pump frequency inthe resonator from the pump photons; preventing transmission ofsubsequent pump photons from the photon source to the resonator whilethe signal photon is in the resonator; and coupling the signal photonout of the resonator in response to a clock signal.
 11. The method ofclaim 10, wherein coupling the pump photons from the photon source tothe resonator comprises: receiving the pump photons from the photonsource using a pump resonator resonating at the pump frequency; andcoupling the pump photons from the resonator to an additional resonator.12. The method of claim 10, wherein generating the signal photon occursvia a degenerate four wave mixing process.
 13. The method of claim 10,wherein preventing transmission of subsequent pump photons from thephoton source to the resonator comprises: detecting an idler photoncreated from the pump photons; and controlling a resonance frequency ofthe resonator based on detection of the idler photon.
 14. The method ofclaim 13, wherein coupling the signal photon out of the resonatorcomprises changing the resonance frequency of the resonator to thesignal frequency in response to the clock signal.
 15. A single-photonsource comprising: a storage resonator to receive pump photons from aphoton source at a pump frequency ω_(P) and to generate a signal photonat a signal frequency ω_(s) and an idler photon at an idler frequencyω_(i) from the pump photons, wherein the signal frequency ω_(s) isdifferent than the idler frequency ω_(i); a detector, operably coupledto the storage resonator, to detect the idler photon and to generate acontrol signal in response to detection of the idler photon; a pumpgate, operably coupled to the detector and optically coupled to thestorage resonator, to prevent transmission, in response to the controlsignal, and to allow transmission, in response to a clock signal, ofsubsequent pump photons from the photon source to the storage resonator;and a signal gate, optically coupled to the storage resonator, toreceive the signal photon out of the storage resonator and, in responseto the clock signal, to couple the signal photon into an output coupler.16. The single-photon source of claim 15, wherein the storage resonatoris a ring resonator.
 17. The single-photon source of claim 15, whereinthe storage resonator is configured to resonate at the signal frequencyω_(s).
 18. The single-photon source of claim 15, wherein the pump gatecomprises: a pump ring resonator, optically coupled to the storageresonator, to couple the pump photons from the photon source into thestorage resonator, wherein the pump ring resonator is configured toresonate at the pump frequency ω_(P).
 19. The single-photon source ofclaim 15, further comprising: a detector gate, optically coupled betweenthe storage resonator and the detector, to couple the idler photon fromthe storage resonator to the detector and block the signal photon fromentering the detector.
 20. The single-photon source of claim 19, furthercomprising: a feedback link operably coupled to the detector and thedetector gate, wherein the detector is further configured to transmitthe control signal to the detector gate, via the feedback link, to turnoff the detector gate.